The invention is generally related to a method for removing ammonia from fly ash. Particularly, the invention provides for the addition of a treating compound to an ammonia-laden fly ash material that causes the evolution and removal of ammonia from the fly ash.
Fly ash is a fine, powdered glassy material recovered from the gases of burning coal during the production of electricity. The micron-sized fly ash particles consist primarily of silica, alumina, and iron, and may contain various other oxides and residual carbon. Fly ash has a number of uses as an additive for different materials. For instance, when mixed with lime and water the fly ash forms a cementitious composition with properties very similar to that of Portland cement and can be used to replace a portion of the cement in concrete. Also, because fly ash consists of very small particulates, the ash may advantageously be used as a filler in plastics.
Fly ash produced from burning coal often contains significant levels of ammonia. The ammonia content results from contacting ammonia vapor with fly ash to minimize the NOx output of coal burning plants. Ammonia is also utilized to reduce SO3 emissions which can produce a “blue plume” effect causing noncompliance of opacity limits. In another application, ammonia or occasionally urea is injected into the flue stream and deposits onto fly ash as ammonium salts that modify the fly ash's resistivity and enhance electrostatic precipitator performance. In all of these examples, typically some portion of the vapor phase ammonia adsorbs or deposits on the fly ash as ammonia or an ammonium salt.
The presence of ammonia within the fly ash causes problems in the storing, handling, and disposal of the fly ash itself. The presence of ammonia is particularly problematic in situations where the fly ash is in contact with water where the ammonia can runoff or leach into the ground. The presence of ammonia also causes particular problems with odor when using the fly ash as an additive in cement mixtures or concrete applications. The high pH environment inherent to cementitious mixtures causes the ammonium salt to under go an acid/base reaction with the accompanying release of ammonia vapor. The ammonia vapor produces a pungent odor that, if excessively high in concentration, represents a potential health issue. The issue occurs primarily during the placement and finishing of concrete, cementitious grouts, or flowable fill containing fly ash contaminated with high concentrations of ammonia. The issue may continue even after the concrete has cured and hardened.
Industry experience has generally established that fly ash containing less than 100 mg ammonia per kilogram of ash (100 parts per million (ppm)) produces little or no odor when used in the production of concrete. However, the addition of ammonia at the power generation plant may results in fly ash ammonia contents of 200 to 2500 ppm, rendering the fly ash unacceptable for use in concrete. Removal of ammonia from fly ash such that the fly ash can be used in concrete would reduce the need for solid waste disposal. It would also benefit the concrete producer by lowering the cost of materials and increasing product quality. Partial substitution of Portland cement with fly ash benefits the environment by reducing the emission of greenhouse gases associated with the production of cement. Removal of ammonia from fly ash may also be desirable for fly ash to be disposed in a landfill or via other disposal methods, and may be required by environmental or other local rules.
Several methods have been proposed and used in the past to remove ammonia from fly ash. Perhaps the most common method is by contacting the ammonia-laden fly ash with water. For example, U.S. Pat. No. 6,077,494 describes a method of reducing ammonia values in ammonia-laden fly ash by contacting the fly ash with water. The evolution of ammonia will generally be enhanced by contacting the fly ash with small amount of alkali such as quicklime (CaO), hydrated lime (Ca(OH)2), sodium hydroxide (NaOH), fluid bed residue, and solutions, slurries, and combinations thereof, along with the water. In practice, it has been found that for low alkalinity fly ash, the alkali source is required when using water for ammonia removal. The fly ash is exposed to a mist or fog of water and is agitated to cause evolution of ammonia. The primary disadvantage to this method of ammonia removal is that after wetting, the fly ash must either be maintained in a wet condition until mixed as cement, or it must be dried by using expensive and energy intensive equipment. In addition, in practice, dosages of the alkali sources referenced above need to be controlled accurately because the presence of excess quantities of free lime in the processed fly ash changes the behavior of the fly ash in concrete, decreasing its usefulness.
Other treatment options involve heating the ash and driving off the ammonia by heat. Exemplary heat treatment methods are discussed in U.S. Pat. No. 6,755,901 and U.S. Patent Publication No. 2003/0205184. The heat treatment options allow the removal of ammonia without the need to add additional chemicals to the fly ash. However, the energy needed to reach and maintain elevated ash temperatures, such as 900° F., and the process equipment required to flush the ammonia gas from the mass of the ash, separate the ash from the ammonia gas, and cool the ash, make this an expensive option.
It is desired to provide an improved method for reducing ammonia content in ammonia-laden fly ash without the limitations or drawbacks associated with previous methods of ammonia reduction, namely high energy costs associated with heating or drying. It is further desired to provide a fly ash material having reduced ammonia content such that expense associated with treatment of a cement admixture to neutralize ammonia is not necessary.